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Work Breakdown Structure and Gantt Charts

Creating an Activity Network Diagram

Backward Pass

Before starting the backward pass process, we should explain the critical path which is the path through the network that results in the latest completion date of the project. If any activity on the critical path is delayed, the completion of the project will be delayed by an equal amount. It is the path with the greatest total duration. Therefore, we can add the amount of time estimated for the duration of each activity to the previous activity to determine which path through the network has the longest total duration. As we will explain below, slack will be zero for all the activities on the critical path.

After we complete the forward pass process for all the activities, we can start backward pass by moving from the last activity to the starting activities. It is a subtractive move through the network from finish to the start. In our exercise, the last activity is M with a one-week duration, an ES of 14 weeks, and an EF of 15 weeks which also indicates the overall duration of the project.

  1. Late Finish (LF) for the last activity M is passed from EF (15 weeks). Then, we subtract LF from the activity duration to find the Late Start (LS). It is (15-1) = 14 weeks (Figure 7.15).
Figure 7.15: Starting backward pass at the last activity
Figure 7.15: Starting backward pass at the last activity
  1. Now, it is possible to compute slacks for each activity. It is the difference between LS and ES, or between LF and EF. Both calculations will generate the same result. For Activity M, it is (14-14) or (15-15), which is zero. Therefore, there are no slacks for this activity. We don't have any flexibility for this activity. We cannot have any delays to start the activity or to finish it. The activities where slack is zero are critical.
  2. Then, we carry back the LS time to the nodes immediately preceding the successor node. K and L inherit 14 (LS) from M, and it becomes LF for both predecessor activities. Then, we subtract LF times from the duration of activities to find the LS for these predecessors (Figure 7.16). The slack for L is (10-10) or (14-14), which is zero. Therefore, L is also a critical activity. The slack for K is (11-8) or (14-11), which is 3. It means that we can wait for an additional three weeks to start K because we need to wait until week 14.
Figure 7.16: Passing LS times to successors as LF times
Figure 7.16: Passing LS times to successors as LF times
  1. At a burst point, as is the case when G is followed by two successors, H and I, we pass the lowest LS time of successors (H and I) to the predecessor activity G as its LF time. Therefore, 6 becomes the LF for Activity G (Figure 7.17).
Figure 7.17: Passing successor LS times at burst points as LF times
Figure 7.17: Passing successor LS times at burst points as LF times
  1. When the backward pass is done for all the activities, we can generate all LS and LF times as well as slack times for all of them (Figure 7.18). Thus, we can determine the critical path where the total slack is zero.
Figure 7.18: Completion of the backward pass
Figure 7.18: Completion of the backward pass

The critical path of this project is B – E – G – I – L – M. It is also the longest path. We need to start and finish all these six activities on their scheduled time not to cause any delay in the overall project. Non-critical paths are:

  1. A – C – F – J – K – M: 1+2+2+1+3+1=10 weeks
  2. A – D – F – J – K – M: 1+4+2+1+3+1=12 weeks
  3. B – E – G – H – K – M: 2+1+3+1+3+1=11 weeks

We should always keep in mind that the WBS is not a schedule, but it is the basis for it. The network diagram is a schedule but is used primarily to identify key scheduling information that ultimately goes into user-friendly schedule formats, such as milestone and Gantt charts. The network diagram provides important information to the project team. It provides information about how the tasks are related, where the risk points are in the schedule, how long it will take as currently planned to finish the project, and when each task needs to begin and end.

Schedules must be communicated to project stakeholders. Generally speaking, stakeholders want to know when the work will be completed. Once the completion date is determined, it is important to confirm whether this date can meet the expectations of the stakeholders, in particular the project sponsor, and internal or external clients. Once timeline commitments have been made, stakeholders must be kept up to date on any delays that will cause deviation from the agreed-upon schedule.